In attempting to cause a cloned gene to be expressed in animal cells for the investigation of its function, it is sometimes difficult to obtain transformant cells due to the expression of said gene interfering with cell proliferation. To avoid such a problem it is necessary to have an expression system wherein the level of expression is low in a non-induced condition and wherein the desired gene expression can be effectively induced.
MMTV is a type of retrovirus or RNA virus. In MMTV-infected cells, the virus occurs as a provirus, namely in the form of a DNA integrated into the chromosomal DNA of the host cells. At each end of the thus-integrated DNA, there is an LTR which is about 1.3 kbp in length and composed of three regions, U3-R-U5. The two LTRs are in the same direction.
The LTR has both a promoter function and a transcription terminator function. In the U3 region of the LTR, there is a region capable of binding glucocorticoid receptor proteins (GRs) and this regulates the RNA synthesis. Thus, when a GR is bound to said region, the LTR promoter functions to initiate RNA synthesis, by which a protein is produced. However, the receptor (GR), when used alone, is incapable of binding; i.e., only the glucocorticoid-bound active form of GR can be bound to the receptor binding site of U3 to activate the LTR.
A system is already known which uses the mouse mammary tumor virus (MMTV) long terminal repeat (LTR) for glucocorticoid-induced expression of foreign genes [F. Lee et al., Nature, 294, 228-232 (1981)].
Since the maximum level of expression is restricted by the number of GR molecules naturally present in the cells, high level induction of the desired gene expression, which leads to large quantity production of the desired product, has not been achieved.